The present invention relates to lightning arresters which provide protection against high voltage transients in sealed tanks, such as electric transformers, and in particular to a switch for enabling such an arrester to be disconnected from outside the tank.
Depicted in FIG. 1 is an electric pole-mount transformer 1 described in commonly assigned U.S. Pat. No. 4,975,797. The transformer includes a tank 2 which contains a coil/core assembly 3, i.e., two cores 4 around which are wound coils 5. The assembly 3 is submerged in an insulative oil 6. Mounted on the assembly 3 is an arrester 7 which electrically interconnects a primary wire 8 and an insulator 9. A ground wire 9A electrically interconnects the arrester 7 with a tank wall by means of a stud 9B welded on the tank wall. The arrester 7 protects against high voltage surges by discharging same to ground via the tank. However, the external ground wire 9A is exposed and may be susceptible to damage, and it is required that a stud be welded to the tank exterior to provide a terminal for the ground wire.
It has also been previously proposed to locate the ground wire internally to the tank. However, it is desirable to be able to test the transformer under high voltage conditions, which requires that the ground connection of the arrester be disconnected. If the ground wire is located within the tank, it becomes necessary to open the tank cover in order to disconnect the ground connection. That results in the seal between the tank cover and tank body being disturbed, which is undesirable.
Depicted in FIGS. 2A and 2B is a prior art arrester disconnector switch 100 which can be used in an electrical transformer tank having a lightning arrester, wherein the ground wire is located internally to the tank. The switch 100 includes an externally and internally threaded gland 104 mounted in the tank wall 102 by means of a locknut 105. A gasket 107 is compressed between the wall and an inside end of the gland 104. A threaded end 109 of a hollow tube 106 is threadedly mounted to the fitting 104. The tube 106 is filled with oil and carries a terminal 108 biased toward the wall 102 by means of a coil spring 111. The terminal 108 is electrically connected to a lead 110 which is electrically connected to the ground terminal of an arrester (not shown). A shaft 112 is slidably mounted within the gland. One end of the shaft 112 carries an electrically conductive piston 114 and a pair of guide washers 116, 118. The opposite end of the shaft is in the form of an externally threaded plug 120, having a hexagonal head 122.
When the shaft 112 is in an extended state (FIG. 2B), the piston 114 is disengaged from the terminal 108, whereby there is no electrical coupling between the ground lead 110 and the tank wall 102. When the shaft 112 is pushed-in, and the threaded plug is attached to the gland, as shown in FIG. 2A, the piston 114 contacts the terminal 108, whereby the ground lead 110 is electrically connected to the tank wall through the piston, shaft 112 and gland 104. Thus, when the transformer is to be tested, the thread 120 is unscrewed, and the shaft is pulled out as shown in FIG. 2B to disconnect the arrester ground.
A shortcoming of such a structure is the need to provide an oil-filled tube 106 and a coil spring-biased terminal 108, which can increase the overall cost of the switch. Also, the operating instructions require that the plug 120 be provided with pipe tape or other thread sealant before screwing the plug to the fitting, in order to seal against oil leakage.
It would be desirable to enable an arrester ground connection to be disconnected without disturbing the tank seal, while eliminating the presence of an externally exposed ground wire, external welded-on stud, an oil-filled tube and a spring-mounted contact within such a tube.